Geo-stationary communication satellites have to be positioned in precisely defined space segments assigned by the International Telecommunication Union (ITU). However, the combined effect of oscillations of a period of 24 hours due to the inclination and eccentricity and the long-term drift of the main longitude leads to an apparent movement of the satellite with respect to its nominal position. Therefore, the satellite has to be controlled under the effect of these perturbations by applying periodic orbit corrections in the most economic manner so that the satellite remains within the so-called station-keeping box. For such a control it is necessary to determine precisely the position and/or movement of the satellite which is usually addressed as ranging.
Due to sophisticated orbit positioning techniques of geo-stationary satellites, the requirements for the accuracy of ranging have increased recently. From DE 198 36 602 A1 it is known to position satellites in a so-called satellite cluster at one geo-stationary orbit position in order to make better use of the narrow orbit space. This means that not just one satellite, but a plurality of satellites have to be controlled in one station-keeping box.
The basic technique for determining the space coordinates of a satellite is based on a trilateration solution as described below. FIG. 1 shows the basic configuration of a trilateration measurement. Three transmission stations 101, 102 and 103 are located on earth, wherein the positions P1, P2 and P3 of each transmission station are known. If the distances d1, d2 and d3 can be measured, then the unknown position P4 of the satellite 104 can be determined. Let di denote the distance of the corresponding measurement of each transmission station and let (x, y, z) and (xi, yi, zi) denote the Cartesian coordinates of the satellite 104 and each transmission station Pi, respectively. Then the following relation holds:di=√{square root over ((x−xi)2+(y−yi)2+(z−zi)2)}{square root over ((x−xi)2+(y−yi)2+(z−zi)2)}{square root over ((x−xi)2+(y−yi)2+(z−zi)2)}−√{square root over ((x−x0)2−(y−y0)2+(z−z0)2)}{square root over ((x−x0)2−(y−y0)2+(z−z0)2)}{square root over ((x−x0)2−(y−y0)2+(z−z0)2)}=fi( q), i=1,2,3  (1)where q=[x, y, z]T is the unknown position vector of the satellite 104. The vector of the distance measurements is expressed as: d= f( q)  (2)
A commonly employed method to solve for q in this non-linear equation is the Gauβ-Newton iterative method. The best estimate of q is iteratively approximated as:
                                                        q              →                        ^                                k            +            1                          =                                                            q                →                            ^                        k                    +                                                    (                                                                            F                      ↔                                        T                                    ⁢                                      F                    ↔                                                  )                                            -                1                                      ⁢                                                            F                  ↔                                T                            ⁡                              (                                                      d                    →                                    -                                                            f                      →                                        ⁡                                          (                                                                                                    q                            →                                                    ^                                                k                                            )                                                                      )                                                                        (        3        )            where F is the Jacobian matrix:
                              F          ↔                =                                            ∂                              f                →                                                    ∂                              q                →                                              =                      [                                                                                                      ∂                                              f                        1                                                                                    ∂                      x                                                                                                                                  ∂                                              f                        1                                                                                    ∂                      y                                                                                                                                  ∂                                              f                        1                                                                                    ∂                      z                                                                                                                                                              ∂                                              f                        2                                                                                    ∂                      x                                                                                                                                  ∂                                              f                        2                                                                                    ∂                      y                                                                                                                                  ∂                                              f                        2                                                                                    ∂                      z                                                                                                                                                              ∂                                              f                        3                                                                                    ∂                      x                                                                                                                                  ∂                                              f                        3                                                                                    ∂                      y                                                                                                                                  ∂                                              f                        3                                                                                    ∂                      z                                                                                            ]                                              (        4        )            
However, in practice not only a trilateration configuration according to FIG. 1, but any other configuration like e.g. a bilateration or a quadrilateration configuration can be used.
Although a bilateration does not yield the full ranging information of a spacecraft, a bilateration still might be useful if only a certain coordinate of the spacecraft is of interest.
Furthermore, a quadrilateration configuration allows the solution for an additional unknown quantity. FIG. 2 shows the practical configuration of a quadrilateration measurement. Let say the transponder delay D of the transponder carried by the satellite is an additional unknown quantity besides the space coordinates of the spacecraft. Typically, such a transponder comprises frequency converters, high-power amplifiers etc. Then the approach for each distance measurement di is as follows:
                                          d            i                    =                                    D              +                                                                                          (                                              x                        -                                                  x                          i                                                                    )                                        2                                    +                                                            (                                              y                        -                                                  y                          i                                                                    )                                        2                                    +                                                            (                                              z                        -                                                  z                          i                                                                    )                                        2                                                              -                                                                                          (                                              x                        -                                                  x                          0                                                                    )                                        2                                    +                                                            (                                              y                        -                                                  y                          0                                                                    )                                        2                                    +                                                            (                                              z                        -                                                  z                          0                                                                    )                                        2                                                                        =                                          f                i                            ⁡                              (                                  q                  →                                )                                                    ,                  i          =          1                ,        2        ,        3        ,        4                            (        5        )            where q=[x, y, z]T is again the unknown position vector of the satellite. The solution for q can be found by rewriting the formulas (2), (3) and (4) accordingly. The Jacobian matrix is then:
                              F          ↔                =                                            ∂                              f                →                                                    ∂                              q                →                                              =                      [                                                                                                      ∂                                              f                        1                                                                                    ∂                      x                                                                                                                                  ∂                                              f                        1                                                                                    ∂                      y                                                                                                                                  ∂                                              f                        1                                                                                    ∂                      z                                                                                                                                  ∂                                              f                        1                                                                                    ∂                      D                                                                                                                                                              ∂                                              f                        2                                                                                    ∂                      x                                                                                                                                  ∂                                              f                        2                                                                                    ∂                      y                                                                                                                                  ∂                                              f                        2                                                                                    ∂                      z                                                                                                                                  ∂                                              f                        2                                                                                    ∂                      D                                                                                                                                                              ∂                                              f                        3                                                                                    ∂                      x                                                                                                                                  ∂                                              f                        3                                                                                    ∂                      y                                                                                                                                  ∂                                              f                        3                                                                                    ∂                      z                                                                                                                                  ∂                                              f                        3                                                                                    ∂                      D                                                                                            ]                                              (        6        )            
A further unknown quantity is the time delay introduced by the receiving arrangement due to unknown delays, e.g. due to error recovery mechanisms.
From WO 00/48018 it is known to use two separate receiving arrangements in one transmission/receiving station to compensate this kind of unknown delays. FIG. 3 shows a corresponding transmission/receiving station with a compensation of the time delay introduced by the receiving arrangement. The transmission and receiving station 301 comprises a multiplexer/encoder 302, a QPSK modulator 303, an up-converter 304 and a satellite antenna 305. Digital payload signals 306 consist of elementary data streams and are fed to a multiplexer/encoder 302 which converts the plurality of digital payload signals into a single digital transport stream, for example according to the MPEG-2 and DVB standards. The digital transport stream is modulated by the QPSK modulator 303 and fed to the up-converter 304 which represents the equipment necessary to convert the output of the QPSK modulator 303 into a signal that can be fed to the satellite antenna 305 for transmission to a transponder carried by the satellite. Typically, such a transponder comprises frequency converters, high-power amplifiers etc.
The output signal of the QPSK modulator 303, i.e. the modulated digital transport stream is also fed to a first receiving arrangement 307. The processor 308 analyses the series of samples to trace a predetermined signal pattern. If the predetermined signal pattern is traced, the processor 308 sends a start signal START to a time measurement circuit 309. Upon receipt of the start signal START the time measurement circuit 309 begins to measure the time until it receives a stop signal STOP.
The stop signal STOP is generated by a second processor 308′ receiving an output signal from a second receiving arrangement 307′. The first and second receiving arrangements 307, 307′ are identical regarding their structure and components. The input signal to the second receiving arrangement 3071 is supplied from a down-converter 310 which receives a signal from the satellite antenna 305 and which comprises all the equipment necessary to convert the received signal from the satellite antenna 305 into a signal corresponding to the output signal of the QPSK modulator 303.
However, since the signal has travelled from the satellite antenna 305 to the transponder carried by the satellite 312 and back, the received signal is delayed.
For generating the stop signal STOP, the second processor 308′ traces the predetermined bit sequence in the output signal of the second receiving arrangement 307′ in the same manner as the first processor 308. Upon detection of the predetermined bit sequence, the second processor 308′ sends the stop signal STOP to the time measurement circuit 309 which stops the time measurement. The measured time corresponds to the double distance between the ground station 305 and the transponder carried by the satellite 312, wherein fixed time delays in the up-converter 304, the satellite antenna 305, the transponder carried by the satellite 312 and the down-converter 310 can be subtracted accordingly. Since two identical receiving arrangements 307, 307′ are provided, unknown delays, e.g. due to error recovery mechanisms, can be compensated accordingly.
FIG. 4 shows a diagram of a transport stream according to the MPEG-2 standard. The transport stream TS is a sequence of packets basically consisting of a header H (4 bits) and a payload P (184 bits). The header H includes synchronisation information (1 bit), various flags (transport error indicator, payload unit start indicator, transport priority, etc.), a payload identification PID (13 bits) and a continuity counter (4 bits) The payload identification PID is required for demultiplexing the individual elementary data streams. An adaptation field is optional, but is transmitted at least every 0.1 s and contains ancillary program data, especially a program reference clock PCR for regeneration of a 27 MHz clock at the receiving side.
Subsequently, the transport stream TS is processed according to different standards depending on the transmission channel. For transmission via satellites, the European DVB satellite standard (DVB-S) may be applied, which defines among other mechanisms convolutional and read-solomon coding as well as additional error control bits to be added to allow forward error correction (FEC) Similarly, European DVB standards exist for terrestrial (DVB-T) and cable (DVB-C) broadcasting.
The predetermined bit sequence in the transport stream TS can be used to generate trigger signals or predetermined signal patterns on the basis of which the delay caused by the travel path from the satellite ground station to the transponder carried by the satellite and back can be calculated. The predetermined bit sequence may be inserted into the transport stream TS at the uplink side, for example as a specific payload P. In order to avoid insertion of additional packets, the program identification PID or part of it may be used as a predetermined bit sequence. Some PIDs must be present in the transport stream TS, but may have a repetition rate which is too high for the purpose of determining ranging information. Then, the PID may be combined with other information of the transport stream header H, e.g. the continuity counter, in order to define a predetermined bit sequence.
On the basis of the configuration according to FIG. 2, each station can carry out the distance measurement on its own. In a next step, the position of the satellite can be calculated on the basis of formulas (5) and (6) as described above in a central processing station. However, in order to provide independent stations, for each station a transmission equipment has to be provided which makes considerable investments necessary.
An alternative solution is the use of a so-called pseudo ranging configuration. FIG. 5 shows the practical configuration of a quadrilateration measurement by pseudo ranging. Pseudo ranging in the sense of the present application is the simple delay measurement of the time elapsed between the transmission of a signal from one location and the reception of the same signal to another location. In practice, one transmission station and a plurality of receiving stations are established as shown in FIG. 5. Preferably, one receiving station is combined with the transmission station. However, in order to determine for each station the transmission delay, it is now necessary to introduce a time synchronization between all stations. Only if the receiver also knows when the transmitter actually has sent the signal, it is possible to measure the delay or so-called pseudo range. In the case of geo-stationary satellite ranging, the delay is still relatively short (about 250 ms), so that the clock synchronization needs to be highly stable for short periods. More specifically, the synchronization accuracy should be at least below 10 ns.
Naturally, the same compensation technique as known from WO 00/48018 can be applied also for pseudo ranging. FIG. 6 shows separate transmission and receiving stations with a compensation of the time delay introduced by the receiving arrangement. The transmission station 601 comprises a multiplexer/encoder 602, a QPSK modulator 603, an up converter 604 and a first satellite antenna 605. Digital payload signals 606 are elementary data streams and are fed to the multiplexer/encoder 602 which converts the plurality of digital payload signals into a single digital transport stream, for example according to the MPEG-2 and DVB standards as described with reference to FIG. 3. The digital transport stream is modulated by the QPSK modulator. 603 and fed to the up converter 604 which represents the equipment necessary to convert the output of the QPSK modulator 603 into a signal that can be fed to the satellite antenna 605 for transmission to the transponder carried by the satellite 614. Typically, such equipment comprises frequency converters, high-power amplifiers etc.
The output signal of the QPSK modulator 603, i.e. the modulated digital transport stream is also fed to a first receiving arrangement 607. The output signal of the receiving arrangement 607 is processed by a processor 608, which traces the output signal for a predetermined signal pattern. If the processor 608 traces the predetermined signal pattern, it sends a first trigger signal EMISSION to a time measurement circuit 609. Upon receipt of the first trigger signal EMISSION, the time measurement circuit 609 registers the time stamp information (the emission time) supplied by a first clock circuit 611.
Furthermore, the receiving station 612 comprises a second satellite antenna 613 and a down-converter 610′ which receives a signal from the second satellite antenna 613 and which comprises all the equipment necessary to convert the received signal from the satellite antenna 613 into a signal corresponding to the output signal of the QPSK modulator 603. However, since the signal has travelled from the first satellite antenna 605 via the transponder carried by the satellite 614 to the second satellite antenna 613, the received signal is delayed.
The output signal of the down converter 610′ is supplied to a second receiving arrangement 607′. The first and second receiving arrangements 607, 607′ are identical regarding their structures and components, i.e. regarding their influences on the processed signal. The second processor 608′ receives the output signal of the second receiving arrangement 607′ and traces the output signal for a predetermined signal pattern. Upon detection of the predetermined signal pattern, the second processor 608′ sends a trigger signal RECEPTION to a time measurement circuit 609′ which registers the time stamp information supplied (i.e. the reception time) by a second clock circuit 611′.
The second time measurement circuit 609′ then transmits the time stamp information (the reception time) to the first time measurement circuit 609 which calculates the signal delay on the basis of the time stamp information received from the second time measurement circuit 609′ and the time stamp information (the emission time) previously registered by the first time measurement circuit 609.
For future telecommunication satellites, it is planned to replace the currently used wide area beams by narrow spot beams.
FIG. 7 shows a satellite carrying a conventional transponder with a global coverage. The antenna of the transponder has a 3 dB beam width of 17.5° so that the visible part of the earth is covered by the wide area beam. Therefore, all transmission stations are located under one wide area beam, wherein configurations according to FIG. 2 or FIG. 5 are possible. However, as already explained, for future satellites it is planned to replace the currently used wide area beams by narrow spot beams.
FIG. 8 shows a satellite carrying a transponder which is connected to an antenna having several narrow beams. By way of an example, a 3 dB beam width of 1.75° is shown. For example, the narrow spot beam 801 may be used as an uplink transmission path, whereas one or more narrow spot beams 802 may be used as a downlink transmission path. However, for these future satellites, which will only have narrow, asymmetric spot beams, the problem is that a ranging station could only transmit, but not receive its own ranging signal, unless it is located at the intersection of a transmit and a receive beam. This means that a configuration according to FIG. 2 is not possible, but only a configuration according to FIG. 5. Furthermore, due to the small spot beam, the spatial separation between the different pseudo ranging stations is severely reduced and thus the precision of the orbit determination. Especially for co-locating multiple satellites in a single orbit slot (satellite cluster), the accuracy will not be sufficient any more.
FIG. 9 shows the effect of reduced spatial separation between different ranging stations. A two-dimensional representation is shown for simplifying matters which can be easily extended to the three-dimensional case. On the left-hand side a configuration is shown with optimum spatial separation. Two stations (station I, station II) are located on earth at a distance given by the baseline b. Each station performs a ranging measurement, either by a two-way configuration according to FIG. 2 or by a pseudo ranging configuration according to FIG. 5. It can be shown that the uncertainty of the measurement is different with regard to the propagation direction. Across the propagation direction there is a relatively low uncertainty of the ranging measurement, whereas along the propagation direction there is a relatively high uncertainty of the ranging measurement. This effect is symbolised by two error ellipses, one error ellipse for station I and one error ellipse for station II. When the two beams of the stations I and II intersect at right angle, the error in the target position of the satellite may be described as a circle which is given by the intersection of both error ellipses. This means that the high uncertainty along the propagation direction is compensated by the low uncertainty across the propagation direction of the other station.
On the right-hand side of FIG. 9, a configuration is shown with a low spatial separation between two different ranging stations I and II. In this case, the angle of intersection is much below a right angle so that the high uncertainty along the propagation direction cannot be compensated anymore by the other station. Therefore, the intersecting error at the target position of the satellite can be described by a common error ellipse of the stations I and II. This results in a high uncertainty of the ranging measurement across the propagation direction of both stations I and II.